If you don't have the rope or the manpower to provide a top rope belay or a fireman belay, a really good method for increasing safety while rappelling is to use an autoblock. An autoblock is a piece of rope or a sling that is wrapped around the brake side of the rappel rope. If the rappeller accidentally lets go of the brake line, the autoblock will cinch around the brake line and halt the rappeller's fall.
The term "autoblock" is kind of ambiguous as it refers to both the knot and the system. As such, you can create an autoblock system with the autoblock knot, a Klemheist (French Prusik), or a Valdotain Tresse.
For an autoblock to work, the rope/sling must be flexible enough to wrap tightly around the rappel rope, and it must be short enough that it does not slide up the rope and interfere with the rappel device.
My personal preference is to use a 12" prusik loop made out of paracord and clip it to the leg loop of my harness. Most people use accessory cord, so my preference is slightly unconventional. These people might object to my recommendation and say that at 550 pounds, paracord is not strong enough to keep a person from falling. However, paracord is plenty strong for this application, and I've used it many times. Beside the fact that there are actually four strands of paracord grabbing the rope (bringing the total pulling strength close to 2,200 pounds), the gripping force required on the brake strand of the rappel rope is only about 20 lbs because the rappel device is holding most of the weight. That means that in this application, paracord has a safety factor close to 100, which is much higher than the industry-accepted value of 7.
As I mentioned above, the autoblock must be short enough that it does not slide up the rope and interfere with the rappel device. Some people elect to extend their rappel device away from their harness with a sling to give themselves extra space between their rappel device and their autoblock.
Before you use an autoblock in the field, I highly recommend testing your setup someplace safe first. Don't just blindly trust that it will work. The video below shows a rappeller falling 60 feet after he let go of the brake line and his autoblock didn't catch.